There have been several arrangements for retaining removable marking elements in writing instrument casings. For example, there is the very familiar push top mechanism in which a marking element is received within an outer casing, the marking element having a compressible spring along the writing end thereof which bears against the writing tip end of the casing, the top end of the marking element engaging a cam arrangement in the top portion of the unit. The top casing and the bottom casing are generally separable at a median longitudinal point thereof and when the parts are disassembled for replacing the marking element, the marking element freely falls out along with the spring in such a way that the spring can easily be lost and the parts become completely disassociated from each other. There is also in the prior art an arrangement wherein a writing instrument casing retains a marking element without the use of a spring at the writing point end thereof, but indeed captures a spring in the retaining cap portion so that the same resiliently engages the end of the marking element and forces it out through the writing point. The Hoffman U.S. Pat. No. 365,747, for example, shows a top member which is spring-loaded axially outward and may be depressed against the resilient spring to force the writing implement outward. The Biro U.S. Pat. No. 2,491,082 is another example where the lower end of the marking implement is resiliently held inwardly of the writing tip and may be forced outwardly again by a top member that presses the same downwardly and holds it into latched position. In addition, there is the propel-repel type of mechanism in which a marking element is threadingly engaged with an inner reciprocating sleeve and which does not need any resilient element to retain the marking element in position since the marking element is physically fastened by the threaded joint. The instant invention improves upon these prior art constructions by providing a positive lock for retaining the marking element within the writing instrument casing, while at the same time permitting easy replacement thereof.